Do the triangle and pour the floor for solar mass.

If you would make the 9' side 8' instead, construction materials would be much more efficiently used. Setting the floor joists doesn't need to be in triangulated sections, either. Just frame the perimeter and use full length joists in metal hangers - and you're good to go.

I would think building it rectangle will save you money in supplies, time and frustration. You should also consider the shape of the roof and materials you will be putting onto it. I recall Mark previously mentioning building things in 4x4, 4x8 or 8x8 dimensions due to the materials you are building with.

Hi! Longtime reader here, not sure if I've ever commented.

I have a similar situation to yours with a tall south-facing window and another facing southwest. I'm planning to build trellises on the walls nearby and a trellis-style "awning" over each window. These would allow in winter sunshine, and they could be covered with outdoor fabric in season if I choose.

But I'm thinking of planting morning glories or another fast climber to shade the outside walls and windows during summer months. Already have a Gold Flame honeysuckle working its way nicely up one wall.

In your case (and I might try this myself!), the trellis/awning combo might be perfect for tomatoes, zucchini, pole beans, or grapevines! I recall how you noticed tomatoes doing better when the plants were sheltered enough not to get drenched by rain.

Anyway, just a thought. Good luck and thanks for your wonderful blog!

Some items of hindsight after building mine some years ago...Try to keep it simple with a rectangle shaped addition and glazing on all three sides if possible, if not, then at least on the south and southwest. Don't really need glazing on the east side in the winter. Plan on external shading in the summer to control heat gain(interior shading will not work as well due to the windows still allowing infrared through into the living area. Perhaps use hinged shutters until plants/vines grow. And calculate overhang (don't forget to include gutters as overhang if used) to help with shading. Use double hung windows to help with any unintended heat gain venting.

Our passive solar house is deliberately sited facing 7 degrees east of south because southern Michigan supposedly has clearer skies in the morning I honestly don’t think it matters that much. I’d keep it simple and go with rectangles and a floor plan that provides the usable space you need.

Have a look at these passive solar wall panels.

From an engineering point of view, the design looks sound to me. Of course with things like this, the devil is often in the details. But the design shown has actually been built.